I have some with over 2,400 working fine. I have a combobox with over 2400 employee names listed in it and it works fine. I pre load the store when the app is first loaded. Then when any form is opened requiring the data for a combobox, it is already cached to the client and loads instantly. This also reduces the calls to the server because the data is loaded once and used many times. I also have a scheduled job on the server that hits the database once an hour and precreates the json string on the server to be called by the client.
Code:
var cb_Owner = new Ext.form.ComboBox({
store: ds_Assoc,
mode: 'local',
displayField: 'assoc',
valueField: 'ao',
triggerAction: 'all',
fieldLabel: 'Assigned To',
name: 'assignedto',
width: 325,
typeAhead: true,
selectOnFocus: true,
forceSelection: true,
disabled: false
});